The invention is directed to a method for the transmission of information packets between a source LAN emulation client LEC of a first ELAN and an LAN emulation client of a second ELAN.
ELAN stands for emulated LAN as described, in particular, in the specification 94-0035R9, "LAN Emulation Over ATM: Version 1.0" of the LAN Emulation SWG Drafting Group of the ATM Forum of Jan. 6, 1995, Bill Ellington, editor. This is thereby an approach of the ATM Forum to the migration of current LANs to ATM networks. ATM thereby means "Asynchronous Transfer Mode", i.e. asynchronous data or, respectively, information transmission methods. LAN is an abbreviation of "Local Area Network". LANs are datagram-oriented local networks that are described in, among other references, the article by David D. Clark, Kenneth D. Progran and David P. Reed, "An Introduction to Local Area Networks" in Proceedings of the IEEE, Vol. 66, No. Nov. 11, 1978, pages 1497 through 1517. LANs are also described in ISO/IEC TR 8802-1, "Overview of LAN-Standards". LANs offer a connectionless service, what is referred to as the MAC service. MAC thereby stands for "Media Access Control". By contrast to this connectionless service, ATM technology is connection-oriented. When the protocols of the higher layers developed for LANs are to be used in emulated LANs on the basis of an ATM network, the properties of the connectionless MAC service must be produced in this ATM network. The LAN emulation according to the aforementioned specification realizes the MAC service in the local ATM network and thus defines a single emulated LAN, called ELAN below. The standard LAN protocols such as LLC, TCP/IP, SPX/IPX or TP/CLNP can be used in this ELAN.
The LAN emulation supports the two most frequently employed AN standards, namely Ethernet according to IEEE 802.3 and Token ing according to IEEE 802.5, whereby three frame lengths are supported given token ring. The addressing of every LAN station ensues on the basis of a destination MAC address that is unambiguous worldwide. For the transmission of information between LANs, the address is handed over from a high layer are. For the description of the information path, token ring LANs employ what are referred to as route descriptors in the frame header in addition to MAC addresses. The frame can be conveyed to the destination within token ring LANs on the basis of such a descriptor.
Only MAC addresses shall be mentioned below.
For emulation of an LAN in an ATM network, the LAN emulation must, among other things, resolve destination MAC address into destination ATM addresses, realize multicast and broadcast, i.e. a distribution of information to as plurality of or to all subscribers, as well as assure the transmission of LAN emulation frames in the proper sequence.
The LAN emulation has a client-server configuration. The client side is called LAN emulation client LEC and the server side is called LAN emulation service. The LAN emulation service is composed of LAN emulation server LES, broadcast-and-unknown server BUS and LAN emulation configuration server LECS. The LAN emulation client receives the destination MAC address from a higher-ranking layer, for example the LLC layer, and must find the corresponding ATM address, in order to subsequently initiate the setup of a direct ATM connection to the destination by signalling. The signalling can thereby ensue, for example, according to the ITU-T Recommendation Q.28311Q.2971. An LAN emulation client can be realized in the software or in the hardware of the stations that participate in the LAN emulation.
An LAN emulation server LES maintains a table with all MAC addresses that are reported in the emulated LAN, for example in the framework of a configuration, and with the corresponding ATM addresses. The communication between the LAN emulation clients and the LAN emulation client ensues according to an LAN emulation address resolution protocol that, conforming to the English designation LAN Emulation Address Resolution Protocol, is referred to as LE.sub.-- ARP. When an LAN emulation client does not know the destination ATM address of a destination MAC address, then it sends an inquiry with the destination MAC address to the LAN emulation server. Such an inquiry for address resolution is referenced LE.sub.-- ARP request. When the LES can resolve the destination ATM address, it replies with LE.sub.-- ARP response. When it cannot, it sends the request to further LAN emulation clients.
When an LAN emulation client receives an address resolution response LE.sub.-- ARP response, then it sets up an ATM-UBR connection to the ATM address contained therein and sends a unicast frame. UBR thereby denotes "Unspecified Bit Rate", i.e. indicates that the bit rate is not specified. A unicast frame is an information or, respectively, data packet with a single addressee. In the transmission of frames within an ELAN, a distinction is made between unicast frame to one receiver and multicast or broadcast frame to several or all receivers. An ATM-UBR connection is maintained for 20 minutes from the last transmitted frame so that further frames can be sent to the same receiver in a simple way. To this end, the variable C12 is referenced in point 5.1.1 of the LAN emulation specification. The destination ATM addresses of destination MAC addresses is stored for a certain length of time in the LAN emulation client with the assistance of a cache mechanism. When there is no connection to a destination LAN emulation client but the destination ATM address is known in the sender LAN emulation client, a sender LAN emulation client LEC can set up a connection without address resolution request and send a unicast frame.
Multicast frames to a group of subscribers or, respectively, LAN emulation clients and broadcast frames to all subscribers or, respectively, LAN emulation clients LECen are sent to the aforementioned BUS. Within an ELAN, the BUS maintains connections to all LEC for the arrived frames to the addressees.
Every LAN can be reported as what is referred to as proxi-LEC. A proxi-LAN emulation client receives all address resolution requests LE.sub.-- ARP request that an LES cannot resolve. A proxi-LEC also receives all multicast and all broadcast frames.
The advantage of ATM technology is to be seen, among other things, therein that direct connections with flexible bandwidth can be set up between the communication parties. Such direct connections guarantee minimum time delays and a high information transmission rate. This advantage of ATM technology is utilized in the LAN emulation for unicast frames. Various concepts for connecting local ATM networks such as, for example, ELANs via a wide-area ATM network are known in the article, "Interconnect Emulated LANs with Wide Area ATM networks" by Peter T. P. Chang and Bill Ellington, ATM Forum Technical Committee of November 29 through Dec. 2, 1994. In a first concept, a plurality of ELANs are thereby connected to a wide-area ATM network, whereby the address resolution and the data transmission are undertaken via a single LAN emulation server and a single BUS. This concept leads to an enormous traffic volume for the realization of the broadcast function. The address resolution delay times in such a network are extremely high.
A further concept provides that ELANs be respectively connected to a wide-area ATM network via remote bridges. Either all remote bridges are thereby connected to one another via permanent virtual circuits PVC or the remote bridges are dynamically connected to one another with the assistance of an ATM signalling upon employment of an address resolution server. The transmission possibilities are thereby limited by the transmission possibilities of the remote bridges and the bandwidth of the permanent virtual circuits between two remote bridges. The remote bridges are flooded with broadcast and unknown servers of remote ELANs insofar as the remote bridge thereof does not respectively know the address of the remote bridges allocated to the destination MAC addresses.
A further concept provides that, instead of remote bridges, routers be provided, a mixture of bridge and router. In this case, these routers fulfill the function of an LAN emulation bridge at the ELAN side and fulfill the functions of a router at the side of the ATM wide-area network. As a result thereof, the broadcast problems are reduced; however, a limitation of the transmission possibilities via the ATM wide-area network due to the transmission possibilities of the routers and of the permanent virtual circuits continues to exist.
A further concept provides that the LAN emulation servers of the individual ELANs as well as the BUS of the individual ELANs be connected to one another by direct connections. This, however, leads to a great plurality of direct connections and to a high traffic volume between the LAN emulation servers and the BUS of the individual ELANs. The traffic volume thereby increases linearly with the plurality of connected ELANs. A further concept provides that the LAN emulation servers of the individual ELANs as well as the BUS of the individual ELANs be connected to a higher-ranking LAN emulation server or, respectively, to a higher-ranking BUS via direct connections. This, however, likewise leads to a great plurality of direct connections and to a high traffic volume. The multilayer nature of BUS and higher-ranking BUS or, respectively, LES and higher-ranking LES also leads to time delays.